Electronics/Metrology/Reliability Laboratory, Swiss Federal Laboratories for Materials Science and Technology, Empa, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.
Ultrasonics. 2013 Sep;53(7):1309-24. doi: 10.1016/j.ultras.2013.03.017. Epub 2013 Apr 18.
Non-destructive assessment of delaminations in glued laminated timber structures is required during their full life cycle. A novel air-coupled ultrasound (ACU) method has been developed, which is able to separately detect delaminations in individual bonding planes of arbitrarily high and long laminated stacks and typically 200 mm wide. The 120 kHz ACU transmitter-receiver pair is positioned at two opposite lateral faces of the sample, with a small inclination with respect to the inspected bonding planes, so that an ultrasound beam is excited at a user-defined refraction angle within the sample, interacting with defects in a limited height portion of the stack. The attenuation of the ultrasound beam transmitted across the defect (negative detection) provided better sensitivity to defects than the scattered fields (positive detection), which are masked by spurious fields. Dedicated finite-difference time-domain (FDTD) simulations provided understanding on the wave propagation and defect detectability limits, with respect to the heterogeneous anisotropic material structure introduced by the curvature of the annual rings in individual timber lamellas. A simplified analytical expression was derived to calculate refraction angles in timber in function of insonification angle and ring angle. Experimental results show that the method is able to detect >20% wide defects in both isotropic material and in glulam with straight year rings, and >50% wide and 100mm long defects in commercial glulam beams. The discrimination of defects from background variability is optimized by normalizing the images with respect to reference defect-free sample sections (normalization) or previous measurements (difference imaging), and by combining readings obtained with distinct ultrasound beam refraction angles (spatial diversity). Future work aims at the development of a tomographic defect inspection by combining the described theoretical and experimental methods.
在胶合木结构的整个生命周期中，都需要对分层进行无损评估。本文开发了一种新颖的空气耦合超声（ACU）方法，能够分别检测任意高和长层压板的各个粘结面中的分层，且层压板的典型宽度为 200mm。120kHz 的 ACU 发射器-接收器对位于样品的两个相对的外侧面上，相对于被检查的粘结面有一个小的倾斜角，使得超声束在样品内以用户定义的折射角激发，与堆叠中有限高度部分的缺陷相互作用。穿过缺陷的超声束的衰减（负检测）比散射场（正检测）对缺陷的灵敏度更高，散射场会被杂散场掩盖。专用的有限差分时间域（FDTD）模拟提供了对波传播和缺陷可检测性极限的理解，这与单个木材薄片年轮的曲率引入的异质各向异性材料结构有关。推导出了一个简化的解析表达式，用于计算木材中的折射角，其与入射角度和环角度有关。实验结果表明，该方法能够检测出各向同性材料和直年轮胶合木中宽度大于 20%的缺陷，以及商业胶合木梁中宽度大于 50%、长度为 100mm 的缺陷。通过相对于参考无缺陷样品部分（归一化）或先前的测量值（差分成像）对图像进行归一化，并通过结合使用不同超声束折射角（空间多样性）获得的读数，可以优化对缺陷与背景变化的区分。未来的工作旨在通过结合描述的理论和实验方法来开发层析成像缺陷检测。
